Block copolymers are well-known in the art and can be prepared by employing a number of syntheses, e.g., multi-functional initiators, radical and irradiation syntheses, mechanochemical syntheses, coupling reactions, alkylene oxide syntheses, and ionic reactions. One of the most versatile laboratory syntheses of block copolymers makes use of polymeric phthaloyl peroxide as the initiator. The first monomer is polymerized at as low a temperature as possible and to a low degree of conversion to give a polymer which, when isolated, contains segments of the polymeric initiator. By dissolving the isolated polymer in the second monomer and polymerizing at a higher temperature, block copolymeric macromolecules are synthesized. The first method of synthesizing block copolymers reported was a photoinitiation study of the vapor-phase polymerization of monomers. A film of poly(methyl methacrylate) was deposited on the walls of an evacuated reaction vessel and then chloroprene vapor is admitted. This was block copolymerized by the unterminated radicals of the polymer that were first formed. The flow method of synthesizing block copolymers has also been employed which consists of subjecting a photosensitized monomer to ultraviolet radiation as it passes through a capillary tube into a reservoir of a second monomer, wherein mixing takes place. Likewise, block copolymers can be synthesized by subjecting a mixture of two compatible polymers to mechanical degradation, by subjecting a mixture of two polymers to mechanical degradation in the presence of a crosslinking agent, by subjecting a polymer plasticized with a polymerizable vinyl monomer to mechanical degradation, or by mechanically degrading a polymer in the presence of oxygen to introduce peroxidic groups that can then be used to initiate block copolymerization at a later stage. Degradative process takes place during mastication, milling, calendering, vibromilling, cavitational ultrasonic irradiation, high-speed stirring, and the like. Condensation reactions have been utilized to couple together polymer molecules containing hydroxyl, carboxyl, amine, thiol and certain esters to give block copolymers which are essentially linear in structure. Also employed in the art are ionic reactions to provide living polymers, e.g., sodium complex of napthalene, when formed in a moisture-free tetrahydrofuran solution, is a stable green ion radical that may be used to polymerize styrene at low temperatures. These "living" polymers are well suited to the synthesis of block copolymers by the addition of a second monomer species which was also polymerized by an anionic mechanism. Poly(styrene-b-isoprene), poly(styrene-b-acrylonitrile), poly(styrene-b-1-vinylnaphthalene) and the like have been synthesized in this manner. Still another method employed provides for the polymerization of ethylene oxide by hydroxyl-containing compounds. Since poly(propylene oxide) has a terminal hydroxyl group, it can be used to initiate the block copolymerization of ethylene oxide to give poly(propylene oxide-b-ethylene).
In the past, the use of mercaptans has been limited. For example, ethyl mercaptan has been used as a starting material for making sulfonal and the lower mercaptans have been employed as odorants for natural gas. 2-Mercaptobenzothiazole is an important rubber accelerator. Mercaptans are useful as oxidation inhibitors, for example, mercaptoacetic acid has been employed in hair waving processes. Methyl mercaptan is now becoming important for the synthesis of amino acid methionine and mercaptans have also been employed as initiators for the polymerization of unsaturated carboxylic acid amides with other ethylenically unsaturated monomers.
As related to polymer preparation, mercaptans have been employed to control the polymerization in the manufacture of resin polymers and rubber compositions where large amounts of dodecyl mercaptan and other higher mercaptans are consumed in such processes.